Color toners are used in a variety of applications by both copiers and non-impact printers. Most of the toners commercially utilized are of the dual component type in which the toning system has larger magnetic carrier beads around which smaller pigmented toner particles attach themselves. The control of the toning system is achieved by a magnetic field associated with an applicator roller. The toner particles attach themselves to the magnetic beads by means of an electrostatic force generated by the electrostatic charge of the toner particle itself. It is that charge which also reacts to the force generated by the latent electrostatic image on the photoconductive cylinder, thus developing the image with toner. The electrostatic charge is generated by the triboelectric charging of the particle, that is the static charge generated by the toner particles rubbing and tumbling against the agitator, the sides of the toner sump, and each other. The triboelectric properties of the particles are determined by the composition of the toner particles themselves. Some particles are coated by surface additives to steer the particles toward the right polarity and the right magnitude of charge. However, any change in toner composition, even if slight, can totally change the charging characteristics, and thus the quality of the print produced.
Using present toners and development systems it is extremely difficult to blend the toners uniformly because of the dependence of the tribo charging of the toner upon the composition of the toner used. To get two formulations to behave together as desired would be luck, rather than a reproducible event. Therefore some systems have introduced multi-color capability by the mixing of primary process colors on the paper. For example the Canon CLC uses four toners, each imaging on a separate pass of the imaging cylinder, then mixing at the paper to form the desired color level. Of course this is expensive and has minimal throughput. The E-Print 1000 by Indigo attempts to do a similar job with liquid toner. In each case, though, four separate color toners must be utilized to create the desired color level with the complication of four developing stations.
It is highly desirable to provide a simple charging and developing system which, by using specific primary colors of toners, would allow pre-blending before charging the system, to create specific levels of color for non-impact imaging of specified spot or highlight color applications. It is particularly desirable to make the quality and uniformity of the color insensitive to slight chemical changes in the toner (from one batch to another). These desirable features are accomplished according to the present invention.
The basic aspects of the present invention are to provide a fluidized bed of toner powders and to apply a uniform charge. This has been very difficult to accomplish in the past, and if the bed is not uniform, with a uniform charge, one color will have the tendency to deplete before the other, thereby changing the color on the printed substrate (e.g. paper). However this is avoided according to the present invention by making the various differently colored powders that are utilized in the fluidized bed so that they have substantially the same physical characteristics, such as resistivity, particle size, and flowability.
According to one aspect of the present invention, a method of applying a designated and specified level of color using a transformed mixture of primary toner colors which create that level of color to the substrate is provided. The method comprises the following steps: (a) Making at least first and second differently colored toner powders having substantially uniform physical characteristics. (b) Introducing the first and second toner powders in desired proportions into a fluidized bed. (c) Uniformly mixing the first and second toner powders together in the fluidized bed. (d) Applying a substantially uniform electrostatic charge to the toner powders in the fluidized bed. And, (e) applying the electrostatically charged mixture of toner powders to a substrate to image uniform and specified non-primary color symbols on the substrate.
Step (a) is typically practiced utilizing primary color toners as the differently colored toner powder, and two or more different powders may readily be utilized. Step (a) is also preferably practiced by making the toner powders so that the vast majority of particles making up the toner powders have a size between about 5 microns and about 25 microns (e.g. between about 10-15 microns). The resistivity of the toner powders is preferably greater than about 10.sup.12 ohm-cm. Step (a) is also practiced by making toner powders having flowability between a predefined minimum and maximum, the minimum being established by empirical means, and the maximum by the flowability that would make handling of the powder in mechanical systems too unreliable.
The method is practiced utilizing a fluidized bed apparatus such as disclosed in co-pending application Ser. No. 07/639,360 filed Jan. 8, 1991, the disclosure of which is hereby incorporated by reference herein. In that system, one or more rotors with a plurality of radially extending sharp points are mounted within the fluidized bed, serving to mix the particles together and also to apply a high, uniform charge to the particles. Typically a sufficiently high D.C. voltage is applied, with sufficient concentration, to breakdown molecules in the vicinity of the source application into individual ionic species, e.g. into positive species, comprising H.sup.+ (H.sub.2 O).sub.n, where n=1, 2, . . . 6. Normally this is accomplished by applying approximately +6.5-+8 kV potential, producing a charge sufficient to associate charges of greater (on the average) than 20 microcoulombs/gram with the individual toner particles.
According to another aspect of the present invention a method of imaging a substrate with a designated non-primary color toner while changing from one toner chemical formulation to another is provided. That method comprises the following steps: (a) Introducing a designated first and second differently colored, substantially uniformly physical property toner powders into a fluidized bed. (b) Uniformly mixing the toners together in the fluidized bed. (c) Applying an electrostatic charge to the toner particles in the fluidized bed. (d) Imaging a substrate with the charged toner particles to produce specified and designated, non-primary, uniformly colored symbols on the substrate. And, (e) accommodating slight changes in the chemical composition of the toners being introduced in step (a) without any change in the resulting imaging.
The invention also relates to a fluidized bed of uniform mixture of toner particles. The bed comprises: A first toner powder of a designated first color and having particles with predetermined physical characteristics and a predetermined charge. A second toner powder of a designated second color and having particles with predetermined physical characteristics and a predetermined charge. And, wherein the physical characteristics and predetermined charge of the first and second toner powders are substantially the same, and substantially uniform. The average predetermined charge of the particles is greater than 20 microcoulombs/gram, the vast majority of the particles have a particle size of between about 5-25 microns, and the particles have a resistivity of greater than 10.sup.12 ohm/cm. The colors typically may be primary colors, and a third toner powder or more of a primary color different than the first and second colors is also preferably provided.
It is the primary object of the present invention to provide a simple and effective method of color imaging for copiers, non-impact printers, or the like, utilizing a system that is insensitive to slight changes in chemical composition, i.e. small changes in chemical composition of the toners being added to the system not making a change in the uniformity of the imaging produced utilizing the toners, such as by utilizing a Moore MIDAX 300 system. This and other objects will become clear from an inspection of the detailed description of the invention and from the appended claims.